1. Field of the Invention
The present invention relates to a mold clamping apparatus, and more particularly, to a mold clamping apparatus having a movable core for use in injection molding for making molded products such as preforms, which clamping apparatus includes hydraulic circuits that enable reduced operating time in which molds are opened and closed.
2. Description of the Prior Art
The mold clamping apparatus of the type having a stationary mold to form a body of molded products, a movable mold which closes with respect to the stationary mold to form an opening in molded products, and a core which is placed in the stationary mold through the movable mold and is movable into and out of the stationary mold includes those employed in rotary type injection machines disclosed in the publication of Japanese Patent No. 2, 832, 263 which is incorporated herein by reference. In such a mold clamping apparatus, a mold opening/closing cylinder effects opening and closing of the movable mold, and the movement of the core into and out of the stationary mold is effected by a differential mold clamping cylinder, which is separate from the mold opening/closing cylinder and is fitted with a booster ram.
Referring to FIGS. 1(A) and 1(B), the above-described conventional clamp apparatus is shown having a machine bed 1, a movable platen 2, a pair of tie bars 3, 3, a pair of mold opening/closing cylinders 4, 4, and a differential mold clamping cylinder 6. The tie bars 3, 3 are vertically mounted on the machine bed 1 and penetrate through the opposite ends of the movable platen 2 which is movable upward and downward. The mold clamping cylinders 4, 4 are arranged between the movable platen 2 and the machine bed 1 and cause the movable platen 2 to move upward and downward as a piston 41 in each of the cylinders moves upward and downward. Arranged on the upper ends of the tie bars 3, 3 is the differential mold clamping cylinder 6 which accommodates a mold clamping ram 5. The mold clamping ram 5, which is arranged above the movable platen 2, is directed downward.
A mold clamping piston 5a is arranged in the differential mold clamping cylinder 6. The inside of the cylinder 6 has a relatively larger inner diameter than the outer diameter of the piston 5a in the area above the mold clamping position, such that the piston 5a contacts the inner surface of the cylinder only in the lower-most region of the cylinder to effect mold clamping. This leaves a communication gap about the piston 5a in the upper region of the cylinder. A booster ram 7 is inserted into the above-described mold clamping ram 5 from the upper end of the mold clamping ram. Arranged on the sides of the cylinder 6 are a bypass 8 which connects the upper region of the cylinder 6 to the lower region thereof and a pair of charge cylinders 9, 9 which communicate with the cylinder 6 at the lower end thereof. Pistons 10, 10 are arranged in the charge cylinders 9, 9, respectively. Also, arranged above the movable platen 2 is a mold clamping platen 11 through which the tie bars 3, 3 are arranged. The pistons 10, 10 as well as the mold clamping ram 5 are connected to the upper surface of the mold clamping platen 11.
In such a mold clamping apparatus, a stationary mold 12 to form a body of the molded products such as preforms is placed on the upper surface of the machine bed 1, and a movable mold 13 to form an opening in the molded products is fixed to the lower surface of the movable platen 2. Also, a core 14 to define an inner space in a preform is arranged on the lower surface of the mold clamping platen 11 such that it can move through the movable mold 13 into and out of a cavity defined inside the stationary mold 12.
Opening of the movable mold 13 with respect to the stationary mold 12 is effected by supplying hydraulic oil to a mold opening oil chamber 42 which is defined by a piston 41 in the lower region of the mold opening/closing cylinder 4, while closing of the movable mold 13 with respect to the stationary mold 12 is effected by supplying hydraulic oil to a mold closing oil chamber 43 which is defined in the upper region of the mold opening/closing cylinder 4.
Opening of the core 14 is effected by supplying hydraulic oil from the booster ram 7 to the mold clamping ram 5. Closing and clamping of the core 14 are effected by supplying hydraulic oil to the differential mold clamping cylinder 6. When inside of the differential mold clamping cylinder 6 is pressurized, the mold clamping piston 5a is pushed downward due to the difference in area between the upper surface and the lower surface of the piston to which the pressure applies. This in turn moves the mold clamping ram 5 downward. Upon this, the hydraulic oil that is present below the mold clamping piston 5a flows through the communication gap, around the piston, to above the piston. Thus, no hydraulic resistive force is exerted while the mold clamping piston 5a is moving downward. After the mold clamping piston 5a fits in the lower-most region of the cylinder 6, however, pressure builds up in the cylinder in the same manner as in a typical mold clamping cylinder. This clamps the molds.
This differential mold clamping cylinder 6 has the characteristics that the high-speed mold closing as well as mold clamping can be effected with a small supply of hydraulic oil. In contrast, in the mold opening/closing cylinder 4, which is of a typical construction and includes the two oil chambers 42, 43 defined by the piston 41, the speed at which the molds are closed and opened is varied depending on the discharge volume of the hydraulic pumps connected to the cylinder. The pump discharge volume needs to be increased in order to make the opening of the mold by the mold opening/closing cylinder 4 faster. Therefore, it has been difficult to shorten the length of the time it takes for the molds to open, or to close, by simultaneously operating the mold opening/closing cylinder 4 and the differential mold clamping cylinder 6, each of which employs different system.
The present invention addresses the above-mentioned problem associated with the differential mold clamping cylinder utilized. Accordingly, it is an object of the present invention to provide a novel mold clamping apparatus in which the hydraulic oil forcibly discharged from one of the differential mold clamping cylinder and the mold opening oil chamber of the mold opening/closing cylinder is delivered to the other as their operating oil in an alternating manner, whereby the simultaneous operation of the two kinds of cylinders, which has been regarded as difficult until now, is realized, leading to reduction in the operating time required for the molds to open and close.
According to the present invention, there is provided a mold clamping apparatus comprising a bed on which a stationary mold to form a body of a product to be molded is placed; a pair of tie bars vertically arranged on the bed; a movable platen having each of the tie bars arranged through the opposite ends thereof, and having on its lower surface a movable mold which closes with respect to the stationary mold to form an opening in molded products; a mold opening/closing cylinder arranged between the bed and the movable platen, wherein the mold opening/closing cylinder moves the movable platen upward and downward to effect the opening and closing of the movable mold, wherein the mold opening/closing cylinder includes a mold opening oil chamber and a mold closing oil chamber, each of the oil chambers defined by a piston, and wherein the mold opening/closing cylinder has a hydraulic line for supplying/discharging hydraulic oil to/from each of the oil chambers; a mold clamping platen having a core movable into and out of the stationary mold through the movable platen; and a differential mold clamping cylinder arranged on the upper end of the tie bars and having a mold clamping ram directed downward, wherein the mold clamping ram has a booster ram inserted there into from the upper end thereof, and wherein the mold clamping ram is connected to the upper surface of the mold clamping platen and has a hydraulic line for supplying/discharging hydraulic oil to/from the booster ram and a hydraulic line for supplying/discharging hydraulic oil to/from the differential mold clamping cylinder; wherein the movable platen is moved upward and downward by supplying/discharging hydraulic oil to/from the mold opening/closing cylinder and opening and closing as well as clamping of the mold clamping platen are effected by supplying/discharging hydraulic oil to/from the differential mold clamping cylinder. In the mold clamping apparatus, the hydraulic line which is in communication with the mold opening oil chamber of the mold opening/closing cylinder and the hydraulic line for the differential mold clamping cylinder are connected to one another via a communication passage, and in that a volume of hydraulic oil which is discharged into the hydraulic lines as the movable mold is opened and closed and as the core moves into and out of the stationary mold is alternately provided to the mold opening chamber of the mold opening/closing cylinder and the differential mold clamping cylinder.
Preferably, a pressure/flow control valve is arranged on the communication passage for controlling the pressure as well as the flow of the hydraulic oil that flows out into the hydraulic lines.
Also, it is preferred that the abed of hydraulic oil that is discharged from the mold opening/closing cylinder is substantially the same as the abed of hydraulic oil that is discharged from the differential mold clamping cylinder.
The present invention, which is characterized in that hydraulic oil discharged from the differential mold clamping cylinder and hydraulic oil discharged from the mold clamping oil chamber of the mold opening/closing cylinder are shared by the two kinds of cylinders as operating oil in an alternating manner, has an advantage that it is not necessary to increase the discharge volume of the pump in order to increase the operating speed of the mold opening/closing cylinder to open and close the mold and the abed of hydraulic oil that is drained is relatively small. Thus, even in a mold clamping apparatus of the type which utilizes a differential mold clamping cylinder operative in a differential manner with a small abed of hydraulic oil to open and close a core as well as to clamp molds and utilizes a mold opening/closing cylinder which is of a normal configuration and requires a large abed of hydraulic oil to open and close a movable mold, the simultaneous operation of the two kinds cylinders, which has been difficult until now, is achieved. Accordingly, it is possible to shorten the molding cycle by reducing the length of the time required for the molds to open and close. Further, the simple configuration of the mold clamping apparatus according to the present invention in which two hydraulic lines, i.e., one for the differential mold clamping cylinder and the other for the mold opening oil chamber of the mold opening/closing cylinder, are interconnected via a communication passage will not make the hydraulic circuit particularly complex and is readily applicable to the existing hydraulic circuits. With a pressure/flow control valve arranged on the communication passage, the mold clamping apparatus of the present invention can be easily controlled. Finally, by making the abed of oil discharged from one of the two kinds of cylinders the same as the abed of oil discharged from the other cylinder, the required abed of operating oil is substantially supplemented by the hydraulic oil discharged from each cylinder. This maximizes the efficiency of the system by minimizing the abed of hydraulic oil that is drained or needs to be added by a pump.